Audio Systems: PA system, worst case scenario, mellow voice

Question
is a 2x100 watt PA enough to play small scale shows? For guitar and vocal?

Answer
That's a loaded question. Here's what you need to know; Generally any decent PA will work for a guitar & vocal, but what I would need to know in order to give you an educated answer is: What is the general size of the room? How many people are in the room? What are the acoustic properties of the room (meaning is it reverberant or dead)? What volume level are you expected to play at(background or foreground)? Is the guitar solid body, hollow body with a microphone, hollow body with a pickup or a combination? What kind of music are you playing? Does the singer have a stong or mellow voice? What is the worst case scenario for a performance? Are you planning to use a foldback monitor?
Are you using a mixer or are you summing the signal into a powered speaker?